In the course of working on Ellie, I discovered something phenomenally silly about the GPX files from my Garmin Vista CX, as uploaded with gpsbabel.
Track log points, quite reasonably, have time stamps in "Zulu time" (essentially the same as GMT, give or take some fraction of a second). They look like this:
<trkpt lat="35.289519913" lon="-115.227057561"> <ele>1441.634277</ele> <time>2010-10-14T17:51:35Z</time> </trkpt>
But the waypoints you set for specific points of interest, even if they're in the same GPX file, have timestamps that have no time zone at all. They look like this:
<wpt lat="35.334813371" lon="-115.178730609"> <ele>1489.917480</ele> <name>001</name> <cmt>14-OCT-10 11:18:51AM</cmt> <desc>14-OCT-10 11:18:51AM</desc> <sym>Flag, Blue</sym> </wpt>
Notice the waypoint's time isn't actually in a time field -- it's duplicated in two fields, cmt (comment) and desc (description). So it's not really intended to be a time stamp -- but it sure would be handy if you could use it as one.
You might be able to correlate waypoints with track points by comparing coordinates ... unless you spent more than an hour hanging around a particular location, or came back several hours later (perhaps starting and ending your hike at the same place). In that case ... you'd better know what the local time zone was, including daylight savings time.
What a silly omission, considering that the GPS obviously already knows the Zulu time and could just as easily use that!
[ 21:09 Nov 03, 2010 More mapping | permalink to this entry ]